A few days ago, my iMac froze on the screensaver. I hard rebooted it. When it started back up, there were vertical stripes on the screen, even over the Apple logo on the startup screen. Then it went to this checkerboard pattern:

Figuring it was a bad video card, I made an appointment to take it to the Apple Store. But today I tried booting again, and it's working fine.

This is a mid-2011 20.5" iMac running 10.7.5. It could be a coincidence, but these issues started shortly after I installed AirPort Utility 6.2 and Digital Camera RAW Compatibility Update 4.04 via Software Update.